### Name: lengthKey
### Title: Key for interpreting lengths in a plot
### Aliases: lengthKey
### Keywords: misc

### ** Examples

 # manufacture a matrix of orientations in radians
 o<-matrix(rep(pi*seq(0.1,0.8,by=0.1),7),ncol=8,byrow=TRUE)
 m<-matrix(rnorm(56)+4,ncol=8,byrow=TRUE)
 # get an empty plot of approximately 1:1 aspect ratio
 plot(0,xlim=c(0.7,8.3),ylim=c(0.7,7.3),type="n")
 vectorField(o,m,vecspec="rad")
 # the scaling usually has to be worked out by trial and error
 lengthKey(0.3,-0.5,c(0,5,10),0.24)



